1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, the invention relates to a technique that is useful when applied to inductors that are incorporated in semiconductor devices.
2. Description of the Related Art
Semiconductor integrated circuits designed for RF (Radio Frequency) use may comprise an inductor in some cases. The inductor is a multi-layer component comprising layers deposited one upon another on, for example, a semiconductor substrate.
Other circuit elements, such as MOS (Metal Oxide Semiconductor) transistors, are formed on the semiconductor substrate. The noise that the other elements generate may influence the characteristics of the inductor. To protect the other circuit elements against the influence of the noise, a guard ring is formed, surrounding the inductor.
FIG. 10 is a plan view showing a conventional semiconductor device that comprises an inductor and a guard ring. FIG. 11 is a sectional view of this semiconductor device, taken along line V—V shown in FIG. 10.
The semiconductor device comprises an inductor 1, a semiconductor substrate 2, and a guard ring 3. The inductor 1 is provided above the semiconductor substrate 2 and comprises a first metal part 1a, a via plug 1b and a second metal part 1c. The first metal part 1a is shaped like, for example, a spiral. The guard ring 3 surrounds the first metal part 1a. The guard ring 3 is a diffusion layer formed by injecting p- or n-type impurities into the substrate 2. An element-isolating region 4 is formed in the surface of the semiconductor substrate 2 and surrounds the guard ring 3.
A potential-applying line 5 is provided above the guard ring 3. The line 5 is almost identical in shape to the guard ring 3. A plurality of contact plugs 6 connect the guard ring 3 and the potential-applying line 5. The line 5 is fixed at, for example, the ground potential. Thus, the guard ring 3 is fixed at the ground potential and set at the same potential as the potential-applying line 5. Note that an insulating layer, for example, is formed between the semiconductor substrate 2 and the inductor 1.
In the semiconductor device thus configured, the inductor 1 is protected from the noise generated by the MOS transistors and the like formed on the semi-conductor substrate 2. Hence, the noise would not influence the characteristics of the inductor 1.
In this regard, techniques of reducing the noise of inductors are known (see the specification of U.S. Pat. No. 5,936,299).
A current may flow clockwise in the inductor 1. In this case, a magnetic flux develops in the inductor 1, which extends backward from the plane of FIG. 10. Further, a magnetic flux develops outside the inductor 1, which extends in the opposite direction, that is, forwardly from the plane of FIG. 10. The flux outside the inductor 1 changes as the current flowing in the inductor 1 varies with time.
The electromotive force V in the semiconductor device is given as follows:V=dφ/dt
where φ is the intensity of the magnetic flux extending in the guard ring 3 and the potential-applying line 5, both provided outside the inductor 1. Thus, the electromotive force is generated in the guard ring 3 and also in the potential-applying line 5. The electromotive force induces a current. The current flows in the guard ring 3 and potential-applying line 5 in the direction opposite to the current that flows in the inductor 1. The inductance of the inductor 1 inevitably decreases. Further, the current flowing in the guard ring 3 and the potential-applying line 5 increases the power consumption. This lowers Q (Quality Factor) of the inductor 1.